Modern broadband wireless networks have to support heterogeneous users with a large variation in service requirements. While the peak data rate of most systems increases with the bandwidth, the number of simultaneous users a system can support does not always increase proportionally due to the system granularity issue. TDMA (time-division multiple-access) and CDMA (code-division multiple-access) are known to have limited granularity and thus suffer from decreased spectrum efficiency when the system is heavily loaded. Other performance parameters that may be affected by coarse granularity include packet delays and jittering.
By partitioning the radio resource in both the frequency domain and the time domain, orthogonal frequency-division multiple-access (OFDMA) offers flexibility and granularity over either TDMA or CDMA. Similar to TDMA and synchronous CDMA with orthogonal spreading codes, each traffic channel in OFDMA is exclusively assigned to a single user, eliminating intracell interference in a system. A salient feature of orthogonal multiple access schemes such as OFMDA, TDMA, SCDMA is its capability to explore the so-called multiuser diversity in a wireless network (R. Knopp and P. A. Humblet, “Information capacity and power control in single-cell multiuser communications,” in Proc. IEEE Int. Conf. Comm. 1995, Seattle, Wash., June 1995, pp. 331-335).
Adaptive coded modulation (ACM) and dynamic channel allocation (DCA) are well-known in the art. For example, for more information on DCA, see U.S. Pat. No. 6,606,499, “Dynamic channel allocation method in a cellular radio communication network.”
When orthogonal multiple access schemes are combined with adaptive coded modulation (ACM) and dynamic channel allocation (DCA), the spectrum efficiency of these approaches their theoretic bounds. As a matter of fact, it can be proved that in terms of total system capacity, OFDMA is indeed optimal in broadband downlink transmission. For more information, see Jiho Jang and Kwang Bok Lee, “Transmit power adaptation for multiuser OFDM systems,” IEEE Journal on Selected Areas in Communication, vol. 21, no. 2, February 2003.
On the other hand, the benefits of ACM and DCA cannot be fully captured without taking into account the actual traffic patterns of the users. For a system with many low and constant-rate users (e.g., voice), the need for individual user based ACM vanishes. High-qualify traffic channels are sometimes wasted, especially in downlink transmission where power control is less feasible. Consider a situation involving a number of voice users with QPSK+½ coding as the default coding and modulation scheme. If only one user is allowed in each traffic channel, the maximum throughput of each traffic channel is fixed as 1 bit/s/Hz, regardless of the traffic channel condition. Note that the problem cannot be solved by buffering (and burst transmission) due to the delay constraints in voice communications.